1. Field of the Invention
This invention generally relates to a ground fault interrupter ("GFI") and, more particularly, to the improvements of desensitzing the interrupter to render the same less sensitive to electrical noise, and of visually indicating with an indicator light the detection of a ground fault condition.
2. Description of the Prior Art
It is well known in the prior art to provide ground fault protection for AC power outlets in consumer and industrial environments. For example, the standard GFI circuit included a differential transformer operative for sensing a current differential between the hot and neutral lines of an AC power supply, and for generating a transformer output signal indicative of the current differential. A fault from the hot line to ground or a fault from the neutral line to ground could be detected. In a common prior art approach, once the transformer output signal was equal to or greater than a predetermined current threshold, a control subcircuit, typically constituted by an integrated circuit control chip, generated an output control signal which triggered a silicon control rectifier into conduction. A solenoid coil was connected to the rectifier and, once the latter was triggered into conduction, a control circuit supply current, which typically measured on the order of amperes, from the AC power supply, was conducted through the solenoid coil to energize the same and, in turn, the contacts of a main line switch connected in the hot neutral lines between the AC power supply and the load were opened. The opening of the main line switch interrupted the hot and neutral lines, and the main line switch was latched in its open state to maintain the hot and neutral lines interrupted, thereby defining a tripped state in which the electrical outlet was protected from ground fault currents until the ground fault condition was corrected. In order for normal operation to ensue, the main line switch was re-set from its open latched state to its closed conducting state to permit a high-amperage line current again to flow from the AC power supply to the load.
In addition, once the ground fault condition was detected, the prior art also proposed the opening of an auxiliary electrical switch in series with the GFI circuit components in order to prevent any electrical power from reaching any of the components when the auxiliary switch was opened, which event occurred substantially simultaneously with the opening of the main line switch. The high-amperage control circuit supply current flowed through the auxiliary switch. Once the auxiliary switch was opened, the control circuit supply current no longer could flow to the GFI circuit components, and the GFI circuit was completely turned off.
Although generally satisfactory for their intended purpose, the prior art GFI circuits were possessed of certain drawbacks. For example, after a ground fault condition, the main hot and neutral lines were interrupted by the opened main line switch, and the GFI circuit was completely turned off by the opened auxiliary switch. Hence, there was no electrical power present to energize a light-emitting element such as an indicator lamp to signify the interruption of power due to a ground fault condition. The signaling of the ground fault condition would have been a very desirable feature, since it immediately visually would alert a user that a ground fault condition existed and should be corrected. At present, most GFI circuits which utilize an indicator lamp energize the latter only during the normal operating condition to signify thereby that no ground fault condition exists. When a ground fault condition, indeed, is detected, the typical prior art GFI circuit which is provided with an indicator lamp turns the latter off.
Also, the aforementioned prior art auxiliary switch was required to be a relatively expensive, heavy-duty switch in order to conduct the high-amperage supply currents therethrough. Even in a GFI circuit which did not utilize any indicator lamp, arcing across the contacts of the auxiliary switch when the same was opened, due to a ground fault condition, was disadvantageous, not only because of the extra expense required for such a heavy-duty switch, but also because of the potential safety hazard and short working lifetime of such switches.
In those prior art GFI circuits which were not shut off and which illuminated an indicator due to the ground fault condition, the GFI components, and particularly the differential transformer, were sensitive to such electrical noise as electrical transients, extraneous current and voltage signals, power supply fluctuations, hash and the like on the hot and neutral lines from the AC power supply. In some cases, it was possible that such electrical noise from the power supply lines could cause the differential transformer to generate a transformer output signal greater than the predetermined current threshold and cause the control chip again to generate an output control signal and re-trigger the silicon control rectifier which, in turn, could cause the solenoid coil again to be energized. The possibility that the differential transformer, the control chip, the rectifier and the solenoid coil again could be activated in response to electrical noise during the tripped state, and that the supply current again could flow through the solenoid coil and overheat the same, meant that these components were at risk and had to be specially designed, which increased the overall expense and made the overall design more complex.